This invention relates to the field of electrical current sensing, particularly but not exclusively for use as a current shunt sensor used in a fiscal utility electricity meter.
Electrical current sensing is used in many applications. There are two common methods used to sense the current. The first method detects and measures the magnetic field produced by the current flowing in a conductor (inductive systems). For example U.S. Pat. No. 4,794,326 describes an arrangement comprising a planar conductor coupled inductively by a transformer arrangement to the sense circuit. A second method of current sensing uses a xe2x80x98shuntxe2x80x99 resistance in the current path to generate a voltage across the shunt resistance in proportion to the current flowing. In U.S. Pat. No. 5,701,253 a utility electricity meter employing a current shunt is described.
In U.S. Pat. No. 4,492,919, a planar current shunt is described. The invention incorporates an electrical transformer to provide isolation between the conductor carrying the current and the sensing circuit.
According to the present invention there is provided a layered current divider comprising a conductor carrying the electrical current and two resistors, all the resistors mounted to occupy substantially the same physical area in a layered structure, wherein the temperature difference between the layers is substantially zero.
The layered structure provides electrical isolation between the main current conductor and the two resistors, whilst providing a thermal conductive path between the layers. This ensures that for any small portion of the main current carrying conductor area, there is a similar area portion on the two other resistors and all three areas are substantially at the same temperature. This ensures that the current divider has substantially zero temperature co-efficient independent of the temperature co-efficient of the metal resistivity comprising the current carrying conductor and two other resistors.
For optimum thermal performance, the main current carrying conductor may be divided into two equal layers. The heat generating main conductor comprises two outer layers and the two other, non-heat generating, resistors comprise the inner layers. The layered structure is isothermal across the thickness.
According to a further aspect of the invention there is provided a current sensor for measuring electrical current flow, comprising a layered current divider and a low input impedance differential current amplifier. The amplifier produces a large signal output directly proportional to the current flowing through the current divider.
One benefit of the current divider when fabricated from PCB is the ability to integrate the current divider with existing electronics manufactured using PCB interconnect technology. The current divider can be integrated directly onto the same PCB used to interconnect the electrical components.
According to a further aspect of the invention there is provided an electricity meter for measuring the electrical power consumed by a load, comprising a layered current sensor, a voltage sensing means and a power computation means.